StealthSonics U9 : The Tightrope Dancer - David Molliere

Pros - Balanced / Reference sound signature - suitable for monitoringVery wide soundstage with pinpoint precise imagingDynamic driver implementation is tight and controlledVery high detail retrieval and separationSuperb top end extensionCoherent tuning, despite the very high level of detail and neutral profile, the U9 is always smooth (no sibilance or harshness)Great value for moneyCons - Lower mids dip means male vocals lack presenceBig shell, protrude significantlyIsolation is only average, even more so with the faceplate offDisclaimer I’ll receive a free StealthSonics review unit among the U series lineup in exchange of my honest opinion on two of the U series lineup.

Listening notesThis review is based on over 20 hours listening through several sources : DX220 with AMP1 mK2 and AMP9 mostly with PW Audio 1960 cable 4 wires unbalanced, AAW Capri lightning cable and iFi iDSD Micro Black Label. I listened mostly with Custom Art custom silicon tips, I had a perfect fit. I did try the U9 with the faceplate off but contrary to the U4 and U2 I didn’t notice much difference, so the review is based on the U9 with faceplate on.

Packaging I didn’t get the U9 packaging for the review tour but based on the U4 packaging I can say the packaging is premium and the carry case is of great quality.

IntroductionThe folks at StealhSonics are a « group of audiologist, engineers and musicians that have been serving the audiology and audio needs of musicians, audio professionals, audiophiles and patients in SouthEast-Asia for almost 10 years ». I confess I hadn’t heard of them before @Jackpot77 (Ross) told me about them, and I am very glad he did and included me in the U series review tour.

Among the U series lineup, the U9 is the flagship offering a hybrid 8xBA and 1xDD array of drivers and is priced just under 1100$ in its universal form. This fall right into the category of products I think are in a sweet spot of TOTL performance with a sane price thus I was very eager to see what StealthSonics brought to the table in the bracket and compared to other TOTL I own and have auditioned.

The U9 is advertised as having a signature that « is supported by a deep bass response, beautifully transparent midrange and crystal-clear open highs that are powered by four Super-High drivers. Crisp transients and an immersive soundstage round out a truly superior listening experience. » Does that hold true?

Let’s see!

Fit & BuildThe U9, as all of the U series, has quite a big shell in its universal form. This could be an issue for smaller ears and even with bigger ears the IEMs protrude significantly like the Solaris although to a lesser extent. This contrast with a shorter than average nozzle length, but that was not an issue for me either with universal tips and custom silicon tips from Custom Art. The build is superb and flawless, with a nice textured finish that provides a high level of comfort. Isolation is only average, it might not be the IEM for a commute depending on the volume you’re listening to (I know I listen at average volume myself). Beware that with the faceplate off, this is even more true and the U9 is leaking a bit as well (only noticeable in a very very quiet environment but forget listening in bed while your spouse is sleeping ).

SoundOverall the U9 signature is balanced and defined around its outstanding clarity and transparency, as well as its resolution. It’s a very neutral IEM that doesn’t add any coloration, it’s neither warm nor bright and tone is spot on. Soundstage is very wide and fairly high and deep, elliptic if you will. There is a lot of air by way of its very good treble extension. Bass is controlled with an emphasis on sub bass, mid bass is not prominent. Mids are tuned for accuracy, lower mids are lean with just enough presence for fullness but the flipside here is the U9 is on the thinner side of the spectrum. Despite being highly detailed the U9 is always smooth thanks to a conservative high mids / lower treble region there was no hint of sibilance or harshness whatsoever.

StealthSonics managed to walk a fine line there and find a good balance and many IEMs including flagships failed at this. Keeping a smooth listening experience with a highly detailed IEM is always a challenge. I found the U9 to be fatigue free despite its very high resolution and detail retrieval abilities. Kudos for staying true to their claim regarding the signature as well, their product page sells it for what it is.

BassStealthSonics has been very disciplined when tuning their dynamic driver with the U9, it’s very apparent they had a clear goal : providing audiophile bass, in the sense that the bass is controlled and detailed, with good texture and good speed for a DD. It won’t give bass-heads a run for their money but it is very high quality and quantity is not lacking either and the snappy attack provides a good bass line and nice rhythm. Decay is pretty fast as well with good recovery time even if it won’t be as fast as a good BA driver.

Interestingly I found bass heavy style to be highly enjoyable on the U9, namely EDM is portrayed beautifully and tracks that usually tend towards messy over the top mid bass become quite articulate. In a totally different genre, I enjoyed strings and especially the double bass on Jazz albums, texture is really good and tone is spot on. There is something reminiscent of JVC FW10000 bass here and it’s high praise in my book.

Mids The U9 mids do not stand out, but in a good way : the U9 is highly transparent. As I hinted, the mids are tuned for accuracy and are neither forward nor recessed.

Interestingly StealthSonics decided to avoid relying on pushing upper mids for clarity and articulation, lean lower mids and upper treble presence does the job already and this would have made the U9 much brighter. This combination grants the U9 a neutral tone, great articulation and separation while remaining absolutely smooth no matter which tracks is played. That’s what I meant when I said mids don’t stand out, so much so that my first impression was all bass and treble, the lack of initial impressions left by the mids was telling.

Vocal placement is smartly put a little forward and vocals are articulate and smooth. The only thing I found myself wishing for is more fullness from male vocals and a tad more body overall but that’s personal preference. The same goes for female vocals, some artists require a bit more upper mids to convey playing on the edge with their performance (think Freya Ridings) and it’s not really there with the U9.

TrebleTreble is probably the first thing you will notice about the U9, because it infuses the whole signature with its upper treble : the ability to provide a very high level of detail, the air in its soundstage and precision in its imaging all comes from there. Extension is top notch, attack and decay are fast, which benefits transient speed.

Paradoxically the U9 is not a treble-head IEM because Stealthsonics decided to play safe with lower treble. The U9 could have been more exciting with more prominent lower treble but it would also have made it more fatiguing and it’s not the case at all as the U9 is fatigue free despite its high detail retrieval. The downside is you could find yourself wishing for a tad more bite, but you can’t have it all.

Verdict StealthSonics meant to build the U9 to be a flagship model designed for anyone who requires accuracy and I have to say, it delivers on its promise. The U9 does have a « deep bass response », a « transparent midrange » and « crystal-clear open highs ». When a product page states something that holds true, you know that the product was developed with clear intent and craftfully executed.

It would have been easy to stray from the goal and add a bit more lower mids to make the U9 fuller or more lower treble to make it more exciting but although I confess I would have liked them to go for it by personal preferences the U9 wouldn’t have held its initial promise or stood apart the way it does (because then it would have been closer to IEMs I know). I do believe while they didn’t state it explicitly one of the design goals was to make the U9 fatigue free and it is.

The U9 features a very interesting tuning because it manages to be neutral and engaging which - like tightrope walking (yeah, that’s where the title comes from ) - is a very hard thing to do. It features the right amount of bass to keep you toe tapping while never compromising on its accuracy goal, the main course is in its bass textures and superb detail and control. It doesn’t feature sparkly lower treble and the excitement that comes along with the added bite. Instead it relies on shimmering upper treble to infuse its signature with refinement and air. Last but not least, the U9 mids have great clarity and transparency completing a very accurate portraying of music whichever genre you choose to listen to. For the U9 is an allrounder that can go from EDM to Classical in a heartbeat with ease. While it will provide a great experience with any genre, you might find yourself lacking some bite on electric guitars on Blues and Rock, and most male singers might not sound as powerful and engaging as some other IEM could portray. You can’t have it all

If you’re looking for a neutral IEM with top notch detail retrieval, great soundstage and accurate tone that provides a fatigue free experience without breaking the bank then the U9 is definitely an IEM you should try! If you’re looking for more bass kick, sweeter mids and sparkle, then other IEMs of the StealhSonics will be of interest.